When one change moves a whole market.

A sector index can be useful without every product in it changing price.

Many software prices stay unchanged for weeks. An index can therefore move in steps when a small number of providers reprice.

Keep the concentration visible.

The sector cards label concentrated movement. That means one listing accounts for at least half the change. The chart remains a valid description of the captured prices, but it should not be read as every supplier moving together.

Explore the aggregate trend and the coverage together. Product-level contributions are not published.